Define accelerated steps time function.

cc/animation/timing_function.cc

Index: cc/animation/timing_function.cc
diff --git a/cc/animation/timing_function.cc b/cc/animation/timing_function.cc
index 7fdb37fed962a48fa68a3e9b58252b694c50f4ca..3c6d4eb8cd3ffbd384d6a97a2c5a78e85d5f489d 100644
--- a/cc/animation/timing_function.cc
+++ b/cc/animation/timing_function.cc
@@ -162,4 +162,29 @@ scoped_ptr<TimingFunction> EaseInOutTimingFunction::Create() {
       0.42, 0.0, 0.58, 1).PassAs<TimingFunction>();
 }
 
+scoped_ptr<StepsTimingFunction> StepsTimingFunction::Create(
+    int steps, bool stepsAtStart) {
+  return make_scoped_ptr(new StepsTimingFunction(steps, stepsAtStart));
+}
+
+StepsTimingFunction::StepsTimingFunction(int steps,
+                                         bool stepsAtStart)
+    : steps_(steps), stepsAtStart_(stepsAtStart) {}
+
+StepsTimingFunction::~StepsTimingFunction() {}
+
+float StepsTimingFunction::GetValue(double x) const {
+    return std::min(1.0, (floor(steps_ * x) + stepsAtStart_) / steps_);
+}
+
+scoped_ptr<AnimationCurve> StepsTimingFunction::Clone() const {
+  return make_scoped_ptr(
+      new StepsTimingFunction(*this)).PassAs<AnimationCurve>();
+}
+
+void StepsTimingFunction::Range(float* min, float* max) const {
+  *min = static_cast<float>(stepsAtStart_) / static_cast<float>(steps_);
+  *max = 1.0;
+}
+
 }  // namespace cc